As a telephone exchange system for homes and small and medium-sized enterprises, there is known a key telephone system including a main device that has a telephone exchange function and to which a public telephone network or a commercial telephone is connected, the key telephone system enabling, by the telephone exchange function of the main device, external communication by each telephone in the system using the public telephone network or internal communication between the telephones in the system.
A recent key telephone system may use various external appliances, and there is, for example, a configuration for enabling communication between a telephone and an intercom in a system by connecting the intercom (an intercom substation installed at the front door, a doorway or the like) to a main device.
An interface circuit for an intercom that may be installed in a main device is described in Patent Literature 1, for example. Also, Patent Literatures 2 to 4 describe adapters for enabling connection of an intercom to a port for connecting a telephone of a main device, for example.
As described, with the key telephone system according to the related art, in the case of installing an intercom, a dedicated connection port for the intercom has to be provided to the main device or a dedicated unit (an adapter) for enabling connection of the intercom has to be prepared.
Now, a configuration for providing a main device with a dedicated connection port for an intercom may be a factor that prevents miniaturization of the main device.
On the other hand, with the configuration for providing a dedicated unit for an intercom, a dedicated connection port for an intercom does not have to be provided, and the system cost for a user who does not need to be connected to an intercom can be reduced. However, in the case where a user who wants to use an intercom cannot secure an installation space for the dedicated unit, the dedicated unit will, in many cases, have to be accommodated inside a main device. Accordingly, a free space, a connector or the like for attaching the dedicated unit have to be provided to the main device, and thus, also with the configuration for separately providing a dedicated unit, miniaturization of the main device may be prevented.